Flow boiling in mini and microchannels offer very high heat tranfer capabilites and they find applicaions in many emerging technologies like electronics cooling and fuel cells. The low flow rate employed in such geometries, coupled with the small channel hydraulic diameter, often results in a laminar flow with all flow as liquid. Since the single-phase flow with all-liquid flow rate is in the laminar range, the flow boiling correlations developed for conventional tubes, larger than 3 mm inner diameter, with turbulent flow need to be carefully reviewed. In the present work, the flow boiling correlation for large-diameter tubes developed by Kandlikar (1990, 1991a) is modified for flow boiling in minichannels by using the laminar single-phase the heat transfer coefficient for all liquid flow. The correlation is also extended for flow boiling in microchannels using the nucleate boiling dominant part of the original correlation. The trends in heat transfer coefficient versus quality are compared in the laminar and deep laminar region. Excellent agreement is obtained between predicted values and experimental data.